Oil was the substance that defined the century just ended; ice will define the one just begun. Frozen in the ice sheets of the globe's high latitudes and the glaciers of its high altitudes are more than 33m cubic kilometres of fresh water. And as the Earth's atmosphere warms, so the cryosphere - that part of the planet constituted by ice - dwindles.

Ice does not go quietly. In 2002, the Larsen B ice shelf in Antarctica, a plate of floating ice the size of Luxembourg, suddenly shattered into thousands of vast icebergs, which then dispersed into the south Atlantic. In the Andes and the Himalayas, glacial meltwater lakes are filling to their capacity, and then bursting. In Spitzbergen, where David Cameron has this week been seeing first-hand evidence of global warming, the glaciers howl as they calve into the ocean.

Across the world, ice is melting at rates unprecedented within human history; melting brought about chiefly by anthropogenic climate change. If global temperatures continue to rise, and the rates of melting continue to accelerate, three main consequences can be expected. Sea levels will rise, leading to massive human displacement from coastal areas and the drowning of cities, provinces, and countries. The great mountain-born rivers that are fed by glacial meltwater - among them the Ganges and the Brahmaputra - and on which hundreds of millions of people in sub-Himalayan Asia and South America depend for their lives, will diminish. And the planet's thermohaline system - the density-driven ocean currents that convey heat around the globe and so maintain our relative climatic steadiness - will be severely disrupted by the vast influx of cold water.

The ice will determine our climatic future; it also contains the records of our climatic past. For ice is a magnificent archivist of weather. It curates meteorological history with peerless care. Glacial and polar ice is formed chiefly by the falling of snow on snow. The weight of the uppermost snow compresses the layers beneath it, which sinter into ice. In the course of this sintering, the ice traps air within itself, in the form of tiny, sealed bubbles. Each layer of ice thus constitutes a tiny atmospheric museum. Climate scientists have cored down into the ice caps, retrieved samples of ice layers that fell as snow hundreds of thousands of years ago and then released and analysed the air contained within the bubbles.

This ice-core analysis has provided many of the most eloquent and extensive data sets on climate change. It was research from Antarctic ice cores, for instance, published in Nature last year, which proved that concentrations of carbon dioxide and methane were far higher now than at any time in the past 650,000 years.

The ice records also tell us something of what we might expect in the coming century, for the planet has gone through climate fluxes before. Our pollution has already committed us to a certain rise in temperature. What is now at stake in climate change politics is how far beyond that committed rise we go. The consequences of a rise of 3C, described last week by the government's chief scientific adviser, Sir David King, as "likely", would be calamitous: a worldwide drop in cereal crops of between 20m and 400m tonnes, 400 million more people put at risk of hunger, and 3 billion people left at risk of flooding and without access to fresh water supplies.

There can be no doubt about it: climate change is the most serious emergency the human species has faced. And to the current frontline of this emergency has travelled David Cameron. Over the past three days, he has styled himself as Cameron of the Arctic, travelling by dog sled over Norwegian snowfields, slip-sliding down into an ice cave, and gazing mournfully at the vanishing glaciers. His trip has been the most eye-catching example yet of his five-month drive to make environmentalism in general, and climate change in particular, the most charismatic features of his new Conservatism.

What is suspiciously unclear, however, is just how Cameron plans to reconcile the strong government intervention necessary to mitigate climate change with the key Conservative chakras of free enterprise, consumer choice and market liberty.

Cameron turned green faster than the Incredible Hulk. For the first five years of his political adulthood, he barely mentioned the environment. Notoriously, he even voted against Gordon Brown's proposed climate change levy, on the grounds of it being a stealth tax on business. And then, on the November 1 last year, during the final weeks of the Tory leadership election, he suddenly went bright green. In a long article in the Independent, he called for an overhaul of public and private life. "Change our political system and our lifestyles: the effects of climate change are being felt right here, right now", the article was bullishly headed.

Since November, Cameron has tricked himself out all over with sprigs of greenery. He has applied to mount a rooftop wind turbine on his Notting Hill house. He has switched his energy supplier to a renewables-only provider. And he has been photographed as often as possible on his bicycle. While these are precisely the sorts of measures that millions of individuals need to undertake to help mitigate climate change, Cameron has made a public piety out of each of them. These acts have, despite the sheen of austerity and self-denial he and his PR team have sought to lend them, been conspicuous consumptions of an odious sort.

All this self-greening has been presented with the hokey mateyness that has become Dave's trademark. "I have a young family, and we share our time between city and countryside," he confided in his Independent article, "so I'm aware of the pressures on the environment in both." Setting aside the smug ease of that phrase "share our time" and the hollow outreach of the "young family" detail, one might observe that a rooftop turbine (the wattage output of which would be sufficient to power the light bulbs in Cameron's house on a windy day) is tokenistic compared with the carbon debit of keeping two houses in different counties and commuting weekly between them.

The reason that Cameron has moved so quickly to mix green and blue - to turn the Tories turquoise - is, of course, in order to soften his party's image. Environmentalism has been chosen as the emollient cream which, when rubbed into the rough face of post-Thatcherite Conservatism, will render it instantly compassionate.

Awkwardly, however, as Blair has found out, the environment is a "soft" issue that demands hard policy choices. Cameron has so far got round this difficulty by a series of astonishingly shameless elisions and contradictions. "The test of a first-rate intelligence," in F Scott Fitzgerald's famous formulation, "is the ability to hold two opposed ideas in the mind at the same time, and still retain the ability to function": the test of a compassionate Conservative, too, it seems.

Consider, for instance, Cameron's extraordinary leadership acceptance speech, in which he declared his desires both "to build more roads" and "to reduce carbon emissions" without any apparent sense of incompatibility. Or what of his December declaration that his new Tory party would work for "liberal values, including a commitment to green policies, localism and deregulation"? How easily he slides there from "green policies" to "deregulation" - how irreconcilable the two are in practice. Such claims are as absurd as Blair's repeatedly stated aim to reduce carbon emissions, even as he continues to subsidise aviation fuel, freeze petrol duty and press ahead with airport expansion plans.

Beyond the inconsistencies within Cameron's own position, there is the question of his inconsistency with the Tory party itself. Let us temporarily assume that Cameron's green vision is genuinely meant, and that he wishes to provide the strong leadership - national and global - on climate change that is so urgently needed. How would this be possible if he came to power? For ranged against him would be the party's multimillionaire businessmen funders, who are unlikely to desire further regulation, as well as the yakuzas of the CBI, the House-Builder's Federation, the British Aviation Authority and other vast lobby groups; while flowing turbulently about him will be the psychopathologies of monopoly capitalism at its mightiest.

Ranged against him would also be many of the members of his own party, unless they have undergone some mass silent conversion over the past few months. We must remember that it was the Tory party whose voting record on green issues was judged by Greenpeace and Friends of the Earth before the European elections in 2004 to be the worst not only in Britain, but the whole of the European Union.

It was the Tory party that privatised the rail network so disastrously and deregulated the buses, thus wrecking the most sustainable forms of transport. Indeed, while Cameron may wish to style himself a new Tory, who is starting his party again from a notional year zero, his road-building pledge alarmingly recalls Margaret Thatcher's deep love affair with automobile and with asphalt. "Nothing can stop the great car economy," Thatcher declared, as she unrolled what her ministers proudly described as "the biggest road-building programme since the Romans".

Can we really imagine a Conservative party advocating the regulation of the influence of supermarkets, the curtailing of the GM industry and the introduction of rigorous environmental taxing? These are precisely the kind of state interventions that post-Thatcherite Conservatism abhors - and precisely the kind of actions which Cameron will need to pursue to make good on his green promises.

Perhaps Cameron is hoping to recover an admirable nineteenth-century Toryism, the Toryism in part of Cobbett, which celebrated a radical localism and an impulse to preserve. But one suspects not: Cameron, like Blair, has no sense of history (this ahistoricism has, dismayingly, been a source of their respective political strengths). Anyway, this brand of genteel Tory paternalism - even if it were recoverable - is many orders of magnitude from what is now required in terms of action against climate change.

Concluding his November article, Cameron declared that the "politician's responsibility" with regard to climate change is clear: "to show the kind of leadership that this government has talked about, but rarely delivered". He was absolutely right. Blair's much-vaunted commitment to climate change has been hopelessly patchy: the political equivalent of trying to stop a glacier melting by draping handkerchiefs on it here and there.

We are entering an era of single-issue politics of a new kind. Climate change is, or should be, the inescapable context for almost all policy decisions. No serious plan for alleviating global poverty, stabilising international tensions, or providing housing, can now be taken independently of it.

Climate change also demands that we think using with entirely new definitions of cause and effect, of culpability, of equity and of political responsibility. It demands, in fact, nothing less than a total restructuring of the human relationship with nature. In these senses, it represents at once the greatest threat that the modern world has known and the greatest opportunity.

This present period of political history will be scrutinised with an exceptional fierceness by future generations, for decisions taken now will have immense consequences then in terms of lives saved or lives lost. The present dilatoriness, squabbling and appeasement may well come to seem thoughtlessly criminal at best, savagely punitive at worst.

David Cameron has, at least, acknowledged the scale of the problem - and of the possible solution. What he must now do is to make some serious and unretractable statements about how he will "deliver" the leadership that would be required to achieve a 90% cut in carbon emissions from the 1990 baseline by 2030. We do not now have the time left to trust to consumer democracy or to technological fixes brought about by the free market. A strong countervailing governmental force is needed to work against both the business state and individual self-interest, and to move us meaningfully towards contraction and convergence.

If Cameron can truly turn his party deep green - if he can establish and then push through the measures necessary to provide a low-carbon future - then he will have done two extraordinary things. He will have contributed towards the saving of millions of future lives and he will also of necessity have rooted out and destroyed Thatcherite Conservatism in all its pernicious forms. A hero's role awaits Cameron of the Arctic - but at present it seems improbable he will assume it.

· Robert Macfarlane is the author of Mountains of the Mind (Granta), which won the Guardian First Book award in 2003. He is currently writing The Wild Places, a book about wildness and the British Isles